I think you're missing something - his comments aren't about either GTK, Clearlooks, or Cairo as shipped with Ubuntu....

No, I got that.

I also got that the the version of Gnome they shipped was supposed to be built on Cairo, and that Cairo's hope of greater efficiency is supposed to come with hardware acceleration. The reviewer noticed that the theme shipped didn't use Cairo and the acceleration wasn't there, so the interface was slow, something at odds with how Cairo and Gnome 2.12 were being reputed.

So, he tried to fix this. This reaction and this result are not surprising given how the version of Ubuntu was configured as shipped.

I was trying to point out that this experience that the reviewer went through should serve as a valuable object lesson to those composing the eventual release of 5.10 - that a moderately informed (by Linux standards) user thought it was slower than expected and couldn't make it faster using the available methods.

"I also got that the the version of Gnome they shipped was supposed to be built on Cairo, and that Cairo's hope of greater efficiency is supposed to come with hardware acceleration. The reviewer noticed that the theme shipped didn't use Cairo and the acceleration wasn't there, so the interface was slow, something at odds with how Cairo and Gnome 2.12 were being reputed. "

No, that wasn't what he said at all. The theme shipped with Ubuntu wasn't slow, it was the theme he DOWNLOADED from CVS that was slow.

Why?
Because it is a development version that uses unoptimised cairo code without any acceleration.

GTK+ as shipped with Ubuntu DOES use Cairo to draw widgets, but no themes currently take advantage of this to make whizz-bang themes. Why? Because it is early code and it is bound to be slow and unoptimised.

Until it is ready, the current generation themes perform perfectly well. And the reviewer did say the desktop had good performance.